This invention relates to a method of operating on a workpiece, for example to modify or prepare a surface topography of the workpiece or parent material in order to increase the degree of mechanical friction or the degree of mechanical interlock with respect to another material or body, or to create holes in the workpiece.
Various methods have been used to increase the surface roughness of materials which have been made relatively smooth by machining, polishing, forming against a flat mould and so forth. These roughening methods include surface deformation by mechanical means such as grooving or knurling, or chemical means such as etching.
Alternatively, material has been added to the parent material surface by means, for example, of arc welding with a consumable filler wire. In one example small drops of metal have been deposited onto a metal surface, utilising the consumable electrode gas metal arc (GMA) process in which the current is modulated such that molten drops are detached from the melting wire electrode in a discrete and regular manner. These drops serve as raised dimples on the otherwise relatively smooth parent material surface and have been used to reduce the tendency to slip between the parent and another body.
EP-A-0626228 describes a method of modifying the surface of a workpiece which is to be joined to another member comprises exposing a series of locations on the workpiece surface to a power beam whereby at each location workpiece material is melted and spaced laterally under the influence of the power beam and then allowed to solidify so as to form a cavity surrounded by an area of solidified material which is raised with respect to the adjacent workpiece surface and has a re-entrant profile.
In accordance with a first aspect of the invention, we provide a method of operating on a workpiece, the method comprising causing relative movement between the workpiece and a power beam in a traverse direction so as to expose a series of locations on the workpiece to the power beam; and at each location causing the power beam to move relative to the workpiece in a predetermined manner having a component transverse to the traverse direction, whereby at each location, workpiece material is melted and displaced by the power beam so as to form a cavity or hole.
The advanced texturing technique claimed here differs fundamentally from the previously known and applied method. In the advanced form, the power beam is specifically caused to move (manipulated) in a predetermined manner at each location, instead of being static. The power beam may be pulsed so as to expose each location, although preferably it is continuous. The typically relatively small, high frequency xe2x80x9csecondary deflectionxe2x80x9d motions of the beam at each location have the effect of markedly altering the redistribution of the molten material generated at each location. In general the movement of the beam is predetermined by an initial step of selecting the desired movement.
The invention can be used to form cavities in the surface of the workpiece or to create holes, typically extending through the workpiece. The formation of a hole can be carried out with or without a backing substrate. Furthermore, when a cavity or hole is formed, preferably the step of causing the beam to move in the predetermined manner is performed after the cavity or hole has been formed. This enables the shape of the cavity or hole to be accurately determined. In many cases a manipulation step may also be used during the cavity/hole formation where the diameter of the cavity/hole exceeds that of the beam.
The secondary deflection is preferably at such a frequency that at least one complete pattern repeat (or at least direction reversal) is performed at each location, and preferably an integer number of repeats is carried out at each position. For 1000 holes per second in steel, a secondary frequency of approximately 1000 Hz, 2000 Hz or 3000 Hz is appropriate. In this case the cavities would be xcx9c0.5 mm in depth and width, on a 0.7 mm pitch. Higher secondary frequencies may be required with smaller diameter cavities, and the material properties also influence the optimum frequency.
The xe2x80x9csecondary deflectionxe2x80x9d used may take various forms, including circular, linear, ellipsoidal and/or simple geometric figure shaped motions, each having at least a component in the direction transverse to the traverse direction. The secondary deflection may also be combined with other suitable variations of temporal and spatial beam power density distribution (e.g. pulsing of beam current and/or varying the beam focus position) to achieve the desired effect and allow finer control over the cavity or hole formation process.
In general, the duration of the beam""s interaction is carefully controlled so that the secondary beam deflection is consistently applied at each location, i.e. the secondary and primary beam motions are correctly phased in relation to one another so that the texturing effect is reproducible and does not vary in a non-systematic fashion.
In the case of cavity formation, suitable raised portions or dimples with re-entrant profiles may be generated on the otherwise comparatively smooth material surface from the parent material by rapidly melting small localised regions by means of a (typically focussed) power beam such as an electron beam impinging on the parent material. The high power density beam is rapidly moved from point to point to produce a series of such raised dimples. By closely spacing the dimples a semi-continuous line or ridge can be produced. It is noted that the raised material comes from melting and displacing the molten material, thus leaving a small cavity in the parent. Some material may be lost by vaporisation under the high power density of the applied beam.
In use, when a workpiece provided with cavities is adhered to another member, both the cavity and the raised dimple can contribute to mechanical interlock.
The adhesion to textured surfaces of materials from the vapour phase is also modified and enhanced.
The re-entrant profile further increases the mechanical forces required to separate the parent from, or slide the parent over, another member.
The re-entrant profile is beneficial in retaining, for example, an adhesive, especially an adhesive which does not itself bond to the parent, or only bonds with a low strength. Alternatively the bond strength between a non-adherent material, e.g. a low friction polymer, and the parent material surface may be increased substantially by the interlocking nature of the re-entrant profile.
In addition, the processed surface is substantially cleaned and freed from any minor contamination by the action of the beam, this increasing the bond strength obtained with any subsequent adhesive bonding operation.
Where a series of closely spaced dimples form a semi-continuous ridge, this ridge preferably also has a re-entrant profile.
For the purposes of increasing the overall bond strength of an adhesive, it is also noted that the adhesive preferably also enters the cavity as well as the re-entrant surface on the dimple to increase its bonding shear strength.
Preferably the surface topography utilises the existing parent material so no additional material is deposited onto the parent. The technique of changing the surface topography relies on utilising the melting/vaporisation capability of a power beam or high power density heat source typically brought to a focus in the region of the work surface.
Preferably a portion of the movement in the predetermined manner causes an area adjacent the location to be heated. Such an area may surround the particular location or may constitute another location. The adjacent location may be heated either before or after the formation of the cavity or hole, so as to effect a pre-formation or post-formation heat treatment respectively. The workpiece surface in the adjacent region can be either heated to below or above its melting point.
The form of the predetermined movement at the location may be similar to that of the portion of the predetermined movement at the adjacent location. However, typically, these have a different form, for example, the portion of the predetermined movement may take a raster form. In another example the area surrounding a cavity is heated by a circular movement of the beam. Typically the heating portion of the predetermined movement occupies about 30% of the time spent by the beam at a location.
The heating of an adjacent location is advantageous in that it can be used to control the cooling rate of the material and in that it produces improved adhesion of molten material to the workpiece surface. Pre-heating also allows the achievement of better synchronisation between the beam movements in the traverse and transverse directions.
The locations are typically spaced such that the material displaced from adjacent locations is in contact. The timing of the beam movements is also preferably controlled such that the material from the adjacent locations remains molten and coalesces during such contact. This coalescence allows a greater range of shapes and sizes to be achieved in the re-entrant features as these features are dependent upon surface tension forces within the ejected material and the temperature (cooling rate). The secondary deflection of the beam is therefore used to control the ejection of material, whereas pre/post-heat treatments can be employed to control the cooling rate.
In the preferred example, the power beam comprises an electron beam. Typically, this will be generated in a conventional manner using a conventional electron gun, the beam being moved across the workpiece under computer control using a technique similar to that described in U.S. Pat. No. 5,151,571 incorporated herein by reference. Typically, the power and speed of movement of the beam is such that more than 500 cavities per second are formed, preferably up to 580 or more cavities per second. The relative speed of movement between the beam and the workpiece is typically up to 1 km/s while the transit time between cavities is typically {fraction (1/100)}th of the dwell time at each cavity. The peak power density of the electron beam is typically in the range 105-107 W/mm2.
Cavities have been made at a range of speeds and sizes. The slowest speed is well below 400/second and there is no practical upper limit to the speedxe2x80x94for example 10000/second can be achieved in some materials, and more than that is certainly possible.
The shape of the beam may be important and preferably the current in the beam annulus is at least half the current in the central portion, and more especially at least equal to the central current. Satisfactory results are obtained when the current in the annulus is twice or even three times the central current, depending on the parent material and type of dimple desired.
In other applications, other power beams could be used, for example a laser. With a laser, typically a slightly higher net energy is required per cavity in contrast to an electron beam, owing to the reduced coupling efficiency with the workpiece. Actual laser pulse durations may be shorter than with electron beams, and correspondingly the peak energy density may be slightly higher. Cavities of similar size can be produced at a rate of 20 per second using a 300W mean output pulsed CO2 laser, giving about 15 joules per cavity.
Potentially any size of cavity or hole can be produced, for example ranging from those with diameters/depths of the order of millimeters to less than 10 micrometers. The minimum diameter of each cavity/hole is determined by the beam diameter, whereas large diameters can be achieved using a large beam diameter and an appropriate secondary deflection. Typically, the holes or cavities have a maximum diameter of substantially 0.6 mm and a depth in the range 0.6-1 mm. In the preferred case, the cavity or hole pitch is about 1 mm.
The dimensions of each cavity or hole set out above will be reduced by the intrusion of remelted material producing a re-entrant feature.
In some cases, the cavities or holes can have a substantially similar form and can then be arranged in either a square or close-packed arrangement. However, cavities or holes of variable size may be produced allowing other patterns of cavities to be formed.
In some cases, textures containing re-entrant and non-re-entrant features could be combined with the production of through holes rather than blind holes. The distinction between this and the known art of EB drilling is in the reliance of the process on its controlled displacement of some or all of the melted material rather than on its complete removal as in classical EB drilling. The control and implementation of this process would be in the secondary beam deflection, to control the timing and extent of the beam""s penetration through the material. Partial ejection of material following penetration would occur, using a volatile backing material in the normal way, followed by a controlled melting and displacement of peripheral material to give the edge profile desired.
Typically, the workpiece is any metal or other material which can be melted or vaporized by the power beam. An example is steel. Non-metals may also be processed even though they are not electrically conductive. For example, re-entrant features can be produced in ceramics such as quartz and alumina, glasses, polymers, and composites. The mechanism is the same, save in some polymers, in which more of the material is vaporised.
In accordance with a second aspect of the present invention, a method of joining a workpiece to another member comprises preparing a surface of the workpiece using a method according to the first aspect of the invention to form one or more cavities in the surface of the workpiece; and adhering another member to the prepared surface of the workpiece.
This method is useful in a number of applications. In particular, the other member can be any polymer or other material which can be successfully introduced by any combination of pressure, heat or chemical reaction into the cavities in a chosen substrate or workpiece. For example, the other member could comprise one of nylon, PTFE, PMMA, aluminium and its alloys, phenolic resins, and magnesium and its alloys.
Examples of important applications of the invention include adhering brake pads to metal brake pad backings; bonding low friction polymers to corrosion-resistant metals for manufacture of prosthetic devices, for example hip joints etc; bonding of rubber to steel or other metals, for example for manufacture of impact-absorbing buffers; and bonding of aluminium to steel or cast iron, for example for the manufacture of lightweight brake disks.